A cheat-sheet guide to the dramatic descent of the Little Rover That Could

NASA

UPDATE August 6, 1:42am EDT: The Mars Curiosity rover has safely landed on the surface of Mars! See the scene at mission control when they got word. Spoiler alert! They were psyched.

Later today, at 10:30 p.m. West Coast time, NASA's Curiosity rover is scheduled to touch down on the surface of Mars. If it's successful, the landing will represent not just a step forward for Martian exploration; it will also represent progress for interplanetary navigation.

But ... if it's successful. People are calling Curiosity's impending landing "seven minutes of terror" for a reason: Though there are many reasons to hope that the landing will go smoothly -- more than 2.5 billion reasons, in fact -- there is also much potential for the touch-down to go spectacularly wrong.

Here's a guide to Curiosity's trip as it's gone so far and as it's expected (and hoped) to conclude.

What is Curiosity, exactly?

The rover is basically a really expensive, and really advanced, robotic photographer. Curiosity -- full name: The Mars Science Laboratory -- is, as our Ross Andersen has put it, "a dune buggy equipped with a set of tools and instruments to shame Inspector Gadget." It carries ten instruments in total, among them two rectangular "eyes" -- the first a primary imaging camera featuring different filters and focal lengths, and the second a large, circular camera that can fire a laser that turns rock into vapor. (Another camera on the rover picks up the images from the laser-firer and interprets their composition.)

So "this rover can go around firing laser beams at rocks and other materials to find out what they're made of; I'd say that's one of its most impressive instruments," planetary scientist Michael Mischnatold Andersen. And the rover can also gauge Martian weather. It can film in HD. And in 360-degree panoramas.

What does it look like?

Curiosity, our latest emissary to the Red Planet, is slightly larger than a standard automobile: It measures ten feet long by seven feet tall at the top of its mast. That makes it substantially larger than NASA's previous Mars explorers, which have ranged in size from slightly-larger-than-a-Roomba to slightly-smaller-than-a-human. And Curiosity really does look like a dune buggy. Here's one picture:

NASA

So, all things considered, Curiosity is actually pretty small! How has that tiny little thing gotten to Mars?

Curiosity has been hitched to an interplanetary spacecraft, tucked inside a protective aeroshell for the duration of the voyage to Mars. It's been traveling, ensconced in that way, for eight-and-a-half months -- and for 345 million miles.

Then begins the descent and landing (EDL) phase -- the true test of Curiosity's mettle. That phase begins when the craft reaches the atmosphere of Mars, roughly 81 miles above its landing site, and ends when the rover -- fingers crossed! -- touches down on Mars intact. This is the "seven minutes of terror" everyone's been talking about: During those tense minutes, the spacecraft carrying Curiosity -- powered by rockets -- will decelerate from its starting velocity (that mind-boggling 13,200 miles an hour at the top of the atmosphere) to a stationary state. The margin of error here is zero: If the craft doesn't decelerate enough, it will hurl into the Martian surface; if it decelerates too much, it'll lose momentum and crash.

And how, actually, will it land?

Right. As the spacecraft is descending, its heat shield will be jettisoned and its backshell will drop away, allowing the craft's built-in radar system to determine its altitude and velocity. That system will leverage the friction of the Martian atmosphere to slow the spacecraft and trigger the craft's robotic steering.

And "that's one of the unique things about this mission -- the capsule isn't just a ballistic object plummeting through space," Mischnatold Andersen. "It actually moves in response to precise conditions in the atmosphere."

To help ensure a gentle landing, The Mars Science Laboratory is making use of the largest parachute ever built for a planetary mission. The device, more than 165 feet in length and more nearly 51 feet across, has 80 suspension lines to keep it secured to its craft. And it's designed to survive the Mach 2.2 punishment it'll take as it hurtles through the Martian atmosphere.

NASA

Part of NASA's challenge here was figuring out how to deal with the (relatively!) large size of Curiosity. "The gravity on Mars is about one-third of the gravity we have here on Earth, which means you don't need as much rocket power to make the spacecraft hover -- but it's still a lot of power," Mischna said. "To get something of this size to the Martian surface, you have to come up with a whole new landing system because the air bags, which we used on previous rovers, just aren't going to work. It would be like dropping a piano wrapped in bubble wrap and expecting it to land without damage. We had to completely rethink our approach to landing, but that's a good thing, because we hope this will be the first of many projects like it, eventually leading to human exploration on Mars. And if you're going to send humans to Mars, you're going to need to bring a lot of heavy equipment -- places to live, food, water, etc."

If you want a first-person sense of the intricacies involved in the landing, there's a (free) Xbox game you can play to give yourself an idea.

Mt. Sharp, in particular -- the official name of which is Aeolis Mons, and which forms the central mound of the Gale Crater -- features an enticing series of layered rock deposits. Those layers could offer scientists a rich source of data about the Martian surface, which is also to say the Martian environment, as it's changed over time.

And there are lots of layers to explore. Mt. Sharp is comparable in height to some of the tallest mountains on Earth:

NASA

And yet the Gale Crater was actually controversial as a landing site, Wired notes. Despite Mt. Sharp's promising layers, some engineers dismissed the site as too risky for a landing. Enhanced navigational procedures, however, made NASA determine that the potential reward offered by the site would be worth the landing's considerable risk.

How will Curiosity actually get free of the spacecraft that's carried it to Mars?

Curiosity, as Mischna noted, is too big to rely on airbags to soften its landing. Instead, the spacecraft itself will use a sky crane to connect the rover to the Martian surface. The method relies on a system of steerable rockets, located on the underbelly of the spacecraft, that will both slow the vehicle and mitigate the effects of any horizontal winds it might encounter. Once the craft has slowed to zero velocity, the sky crane system will lower Curiosity on a bridle that will extend about 25 feet to the ground. (Curiosity will actually, at first, be attached to its craft via three nylon -- nylon! -- tethers, along with an "umbilical cord" to provide power and a communication connection.)

The craft's landing system will place the rover on its wheels, allowing mission controllers to check its systems and make sure that it survived the landing intact and ready to roll. As soon as touchdown is detected, though, the bridle will be cut using pyrotechnic cutters -- and the craft itself will launch away from Curiosity's landfall site, crash-landing at a safe distance from the rover.

NASA

Will the landing really be as dramatic as everyone is saying?

Yes. Mostly because there are so many ways that it could go wrong, and so many aspects of Curiosity's fate, at this point, that are out of human control. Curiosity's planned Martian descent is "the most daring landing ever attempted on an alien world," as The Guardian put it, because the descent itself is alien to us. All we can do, at this point, is watch.

Whatever you see, though, won't be happening in real time: For this mission, there's a 14-minute communications gap between Earth and Mars, meaning that even the scientists who created Curiosity will be spectators in the landing itself. The landing's success, or failure, will be determined by the interaction between earthly computers and Mars's gravitational pull. And while the landing itself is scheduled to occur at 1:30 a.m. Eastern, officials have cautioned that confirmation of the touchdown could take several hours -- or even several days. During which time, we'll just have to wait. And hope.

So what if something goes wrong?

Something could go wrong! And really easily. There are multiple steps that coordinate iteratively over the course of Curiosity's "seven minutes of terror." If any of those doesn't go precisely according to plan, the entire mission will be moot. "Failure could set back American-led Mars explorations for years," The New York Timesnotes.

Then again, though: If Curiosity fails, it won't be the first time NASA has experienced a setback in its overall mission to Mars. In 1999, infamously, the agency lost a Mars orbiter due to confused measurements between metric and English systems. But NASA has also had great successes with rover exploration of the Red Planet. The rover Opportunity, launched in 2003 and landed on Mars in 2004, is still actively exploring the Red Planet. In 2010, NASA finally lost communication with Opportunity's twin rover, Spirit, which, in 2009 -- after nearly five years of Martian recon -- became stuck in Mars's soft soil. So if something goes wrong with Curiosity, "we'll pick ourselves up, dust ourselves off," Doug McCuistion, NASA's Mars exploration program director, told reporters at a news conference on Saturday. "This will not be the end."

What's Curiosity going to do once it's landed?

The rover will ascend up Mt. Sharp, probing its layers for data suggesting that Mars once supported life. "We're looking for habitability, not specifically for life, but for the conditions where life might have thrived," UK Space Agency research fellow Matthew Balme told the Daily Telegraph. He added: "We want to find evidence of standing water." Project scientist John Grotzinger echoes that caveat. "Curiosity is not a life detection mission," he told The Guardian. "We're not actually looking for life; we don't have the ability to detect life if it was there."

Curiosity is equipped with two lithium-ion batteries. But its journeys will also be powered by a plutonium-238 fueled electrical generator -- a lump just the size of a marshmallow. And that little lump, actually, makes Curiosity "revolutionary." Because the Little Dune Buggy That Could has, for the first time, a nuclear power source. The decay of the plutonium, Mischna explains, will create heat that will, in turn, generate electricity that will charge the rover's batteries. And that will produce much more power than any of the previous rovers enjoyed. As an additional edge over solar panels, it will also make environmental factors like clouds and dust irrelevant, giving Curiosity a constant stream of energy.

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